The present invention relates to a gas control system for a spectrophotometer, which is particularly adapted, among other possible uses, for use in atomic absorption spectroscopy.
In atomic absorption spectroscopy, the measurement of the absorption of a radiation beam at a characteristic resonant spectral line for a particular element yields a measure of the concentration of that element in an original sample solution. Presently, one of the most common techniques for atomizing an element for purposes of the absorption measurement, is by introducing a liquid sample solution of the element of interest into a gas burner wherein droplets of the solution are vaporized and the elements ultimately atomized, so as to form in the path of the apparatus radiation beam, a substantial quantity of the element of interest in its atomic state. A sample light beam, which originates from a line-emitting light source, and which includes a resonance line of the element to be measured, is directed through the furnace. The desired element in the sample absorbs the resonance lines characteristic of the element and the emerging light beam is directed to a monochromator and thence to a detector which measures the degree to which the desired element absorbs the resonance lines of the sample beam. This absorption degree represents the amount of desired element in the sample substance.
In some installations, a double beam from the light-emitting source is employed, and in other installations a second double beam from a light source having a continuous spectrum is used, with optical beam-switching choppers to sequentially direct the various beams through the system. In other installations, electrically pulsed operation of the signal and background source is effected in lieu of a mechanical chopper.
Difficulties were experienced with prior art such instruments, when the flame went out, but the fuel and oxidant were still being supplied to the burner. Attempts were made to reduce the hazard by positioning an ultraviolet detector within the burner chamber. The detector was then relied on to indicate the absence of a flame and send out a signal so that the supply of fuel oxidant could be terminated. However, such detectors frequently produced signals, which erroneously represented the presence of a flame when in fact, none existed. Such signals were created by the presence of reflected light, for example, from the measuring beam or from some other outside source, within the chamber. One technique developed to reduce such erroneous signals resided in reducing the light entry aperture of the UV detector. This, however, was not entirely effective. Another technique was to point the aperture of the detector in a direction, which reduced the number of reflections reaching the aperture. This, also, was not entirely effective.
It will be appreciated that the above-mentioned hazardous condition was further aggravated when the instrument operator attempted to improve the sensitivity of the instrument by utilizing a relatively lean gas mixture. In such instruments it was often necessary to install an override feature, which electrically isolated the detector to prevent it from giving off any signals, during the time when lean gas mixtures were being used. This, of course, completely eliminated this safety feature during such operation.
It is, therefore, an object of the present invention to provide an apparatus, which more accurately determines the absence of a flame from a gas burner in a spectrophotometer.